State-of-the-art semiconductor devices are fabricated with reduced feature sizes, and with large numbers of components within a small area of a semiconductor device. Reduced feature size is achieved, in part, by reducing the size of electrical contracts and the junction depth of doped regions within the substrate below the contacts. To fabricate smaller contacts, while maintaining low contact resistance, numerous metal layers are necessary to form the contact structure. The various metal layers are employed to prevent the inter-diffusion of materials present in the metal layers, and to provide adhesion of the metal layers to the underlying substrate.
Historically, metal contacts have been made with aluminum or aluminum alloy metallization. Aluminum, however, presents problems with junction spiking, which results from dissolution of silicon from the substrate and into the aluminum. This problem is exacerbated with small contact sizes because the shallow junction is easily shorted, and because the amount of silicon available to satisfy the solubility requirements of the aluminum is only available through the small contact area. Adding silicon to the aluminum reduces the severity of this problem, but, in turn, has resulted in silicon precipitation and other problems.
Refractory metals have been increasingly employed in high-aspect-ratio contact openings, such as via structures, and the like, to overcome the problems inherent with aluminum metallization. For example, tungsten can be deposited into a contact opening by chemical vapor deposition (CVD). The tungsten is deposited by the reduction of tungsten hexafluoride (WF.sub.6) with hydrogen. However, the tungsten hexafluoride initially reacts with the silicon surface consuming silicon from the contact region. This results in the formation of "worm holes" or tunnels in the silicon, which can cause shorting of the underlying junction. The use of other methods for depositing tungsten, such as sputtering, instead of CVD deposited tungsten, overcomes the problem associated with the use of halogenated tungsten; however, sputtered tungsten is non-conformal and therefore cannot be used to form electrical contact structures, such as via plugs and the like.
To avoid the undesirable effects of halogenated tungsten used in the CVD deposition of tungsten metal, other metals, such as titanium nitride and titanium-tungsten, have been used to prevent unwanted reactions between halogenated tungsten and the underlying substrate material. However, materials such as sputter deposited titanium nitride and titanium-tungsten are limited by the inability to deposit these materials in a conformal manner. The conformal deposition of a barrier metal in a high-aspect-ratio via structure is essential to prevent the reaction of halogenated tungsten with the substrate in to the corners of the via structure. Additionally, titanium-tungsten and titanium nitride increase the electrical resistivity of the contact structure. It is especially desirable to avoid the inclusion of any high resistivity metal in a contact structure. To obtain high electrical performance in a semiconductor device, the contact structures must not create an electrical resistivity higher than that of the electrical interconnect leads used to electrically couple the various device components. Accordingly, further development of contact metallization is necessary to provide interlayers and diffusion barrier layers having conformal deposition characteristics, while avoiding an increase in the electrical resistivity of the contact structure.